1. Field of the Invention
Embodiments of the invention relate to methods of preparation of supported monolithic gold (Au) catalysts that are used for generating a hydrogen-rich gas from gas mixtures containing carbon monoxide, hydrogen and water via a water gas shift reaction, and for the removal of carbon monoxide from air at a low reaction temperature via its oxidation reaction. More particularly, embodiments of the invention include methods for the preparation of highly dispersed gold catalysts on washcoated monoliths and the stabilization of monolithic catalyst supports by the addition of a third metal oxide, such as zirconia (ZrO2), lanthanum oxide (La2O3), or manganese oxide (MnxOy). The catalyst supports and/or washcoats may include a variety of transition metal oxides such as alpha iron oxide (α-Fe2O3), cerium oxide (CeO2), ZrO2, gamma alumina (γ-Al2O3), or their combinations.
2. Technical Background
Hydrogen (H2) or H2-rich compositions are an indispensable feedstock for many chemical and energy-generating processes, including the refinery processes in petrochemistry and the production of chemicals such as ammonia and methanol. These compositions are also applied in proton exchange membrane (PEM) fuel cells for generating electricity, etc. (see W. Ruettinger, O. Ilinich and R. J. Farrauto, J. Power Sources, 2003, 118, 6-65).
H2 is an environmentally benign material as it is non-poisonous and the use of H2 in many energy-generating processes, such as that in (PEM) fuel cells, does not emit any pollutant, as the sole product of H2 oxidation is water. The use of fuel cells complement environmental regulations that are getting increasingly stringent, whereby cleaner, renewable and non-pollution processes and products are demanded. Moreover, in addition to the growing global demand for H2 production and distribution, the need to obtain higher H2 purity is also increasing. For example, the maximum allowable carbon monoxide (CO) concentration in the H2 gas feed of a PEM fuel cell has dropped from a value of 100 ppm, as required in the 1990s, to the current value of 10 ppm or even 1 ppm. The presence of CO could poison the platinum (Pt) electrodes in the Fuel cells (see W. Ruettinger, O. Ilinich and R. J. Farrauto, J. Power Sources, 2003, 118, 6-65).
H2 is commonly produced by splitting water and steam, reforming of fuels (methane or mixtures of hydrocarbons), or as a by-product of steam reforming of naphtha. The former process is very energy-consuming and is not applicable for large scale production unless cheap electricity is available. Therefore, in industry, most H2 (above 90%) is produced from the steam reforming process, which produces a synthesis gas comprising H2, carbon dioxide (CO2), and CO. Using methane (CH4) as an example, the reaction can be expressed as:2CH4+3H2O→7H2+CO+CO2 The CO concentration is usually above 10% in the synthesis gas, thus, it is still not directly applicable in a fuel cell or in the production of ammonia, etc.
To separate H2 from CO, a reaction called the Water Gas Shift (WGS) reaction is used in industry to convert CO into CO2:CO+H2O→CO2+H2 
The current industrial WGS process is composed of a two-stage reaction: the high temperature (HT) WGS reaction, using ferro chromium (FeCr) as the catalyst at a working temperature in the range of from 350° C. to 500° C., and the low-temperature (LT) WGS process, using copper zinc (CuZn) as the catalyst at a working temperature in the range of from 200° C. to 260° C. After the steam reforming reaction of a fuel such as methane, the CO concentration in the synthesis gas can be lowered to about 2% to 4% after the HT-WGS reaction, and further lowered to a value between 0.1% and 1% after the LT WGS process (see L. Lloyd, D. E. Ridler, M. V. Twigg, in: M. V. Twigg (Ed.), Catalyst Handbook, seconded, Wolfe Publishing, Frome, 1989, pp. 283-338). This means that the CO conversion in the latter process should be above 50% to 95%, if the CO concentration is around 2% after the HT-WGS reaction. Since the WGS reaction is exothermic and reversible, it is not favorable to reach such a low CO concentration at high temperatures, in accordance with thermodynamics.
Both the FeCr and CuZn catalysts need to be reduced or activated in situ before use (see W. Ruettinger, O. Ilinich and R. J. Farrauto, J. Power Sources, 2003, 118, 6-65). Also, they are pyrophoric after activation, i.e. they spontaneously generate heat to dangerously high temperatures when exposed to air. Unfortunately, residential or mobile fuel cell systems are often operated with frequent start/stop operations. Therefore, the existing industrial FeCr and CuZn catalysts clearly cannot meet these dynamic requirements, as they will then experience unacceptable rapid deactivation under these operation conditions due to their pyrophoric nature (see R. J. Farrauto et. al. Catal. Rev. 2007, 49, 141-196; and R. Borup, J. Meyers et. al. Chem. Rev. 2007, 107, 3904-3951). In addition, in the production and handling process, as well as the post-reaction treatment, of the chromium-promoted iron catalyst, the presence of chromium is a potential danger to health, and a problem to the environment.
Other developed catalyst systems usually need a higher reaction temperature to convert CO into CO2 via the WGS reaction. For instance, U.S. Pat. No. 5,030,440, reports a Pt and Pt-containing catalyst formulation which needs a reaction temperature above 550° C., while U.S. Pat. No. 5,830,425 discloses a chromium-free iron/copper catalyst that requires a reaction temperature above 300° C. Some other metals such as cobalt (Co), ruthenium (Ru), Palladium (Pd), Rhodium (Rh) and nickel are also tested for WGS reaction, and methanation of CO (CH4 formation from CO and H2) is usually observed as described in U.S. Pat. No. 7,160,533. Furthermore, all of these catalysts cannot fulfill the requirement for frequent stop/start operations.
Therefore, in recent years, research efforts have been made to further lower the WGS reaction temperature, while maintaining the high catalytic activity. It is not only because the low reaction temperature favors a higher CO conversion, according to the thermodynamic equilibrium of the WGS reaction, but also the necessity to lower the temperature of the H2-stream fed to a PEM fuel cell to be as close as that of the operation temperature of the PEM fuel cell, which is around 80° C. Also, as mentioned above, the new WGS catalysts should be able to endure the repeated start/stop operations.
It is reported that supported Au catalysts on α-Fe2O3 exhibited good WGS activity in the temperature range of from 120° C. to 200° C., at a gas hourly space velocity (GHSV) of 4000 h−1 (see D. Andreeva, V. Idakiev, T. Tabakova, A. Andreev, J. Catal. 158 (1996), 354-355). Some other supported Au catalysts on CeO2, TiO2 and ZrO2 were later reported to be used in LT WGS reaction (see D. Andree, Gold Bulletin, 2002, 35, 82-88; H. Sajurai, T. Akita, S. Tsubota, M. Kiuchi, M. Haruta, Appl. Catal. A: General, 2005, 291, 179-187; Q. Fu, H. Saltsburg, M. Flytzani-Stephanopoulos, Science, 2003, 301, 935-907). The use of mesoporous catalyst supports such as mesoporous ZrO2 can further improve the catalytic performance of the catalysts (see V. Idakiev, T. Tabakova, A. Naydenov, Z. Y. Yuan and B. L. Su, Appl. Catal. B. Environ. 63, 178 (2006); V. Idakiev, T. Tabakova, K. Tenchev, Z. Y. Yuan, T. T. Ren and B. L. Su, Catal. Today 128, 223 (2007). A recent U.S. Pat. No. 7,375,051, describes Au supported on sulfated ZrO2 for WGS reaction. At 200° C. and at a GHSV of 4000 h−1, the CO conversion could reach 96%, and after a 20 hr reaction, it was slightly decreased to about 95%. Clearly, the Au catalyst is promising for WGS reaction at low reaction temperatures. Other examples can be seen in US Patent Application No. 2006/0128565A1, where Au/Lanthanum oxides were used for WGS reaction.
The methods developed for the preparation of the supported Au catalysts include the co-precipitation (CP) method and deposition-precipitation (DP) methods (see M. Haruta, S. Tsubota, T. Kobayashi, H. Kageyama, M. J. Genet, B. Delmon, J. Catal. 1993, 144, 175) in which HAuCl4 is used as the precursor, and sodium carbonate (Na2CO3), sodium hydroxide (NaOH) or urea (see R. Zanalla, S. Giorgio, C. R. Henry and C. Louis, J. Phys. Chem. B. 2002, 106, 7634) as the precipitating agents respectively, chemical vapor deposition method (see M. Okumura, S. Nakamura, S. Tsubota, T. Nakamura, M. Azuma, M. Haruta, Catal. Lett., 1998, 51, 53-58), and a Au-colloid-based two-stage method employing tetrakis(hydroxmethyl)-phosphonium chloride (THPC) as the reducing and capping agent (see D. Grunwaldt, C. Kiener, C. Wogerbauer, A. Baiker, J. Catal. 1999, 181, 223-232), and a sonicated-assisted method (Z. Zhong et al, Patent Application No. WO2007/055663 and Z. Zhong, J. Lin, S. P Teh, J. Teo, F. M. Dautzenberg, Adv. Funct. Mater, 2007, 17, 1402-1408), etc.
One factor that determines the catalytic performance of the supported Au catalysts for CO oxidation is the Au particle size. The Au particles should be below 5 nm, and for CO oxidation, the optimum Au particle size should be about 3 nm (see M. Haruta et al, Catal. Lett. 1997, 44, 83; M. S. Chen, D. W. Goodman, Science, 2004, 306, 252). The above methods in most cases can control Au particles below 5 nm. A number of supported Au catalysts (in powder form) have been prepared for CO oxidation (or selective oxidation of CO in the presence of H2) by these methods such as those described in US Patent Application No. 2006/0165576, which reported the use of Au supported on Al2O3, Fe2O3 etc, in the selective oxidation of CO; US Patent Application No. 2007/0190347 A1 which describes Au/CeO2 catalyst applied in CO oxidation in air for treating tobacco smoke; and WO 2007/055663 A1 which also describes supported Au catalysts used in CO oxidation.
However, for these powder catalysts, when they are packed into bed particulate form, a large pressure drop will be created during the catalytic reaction, thus limiting a high speed flow of reactant gases. Moreover, the catalyst beds are vulnerable to severe breakage resulting from stresses that are induced by the frequent start and stop operations required for mobile fuel cell systems (see R. J. Farrauto, Y. Liu, W. Ruettinger, O. Ilinich, L. Shore, T. Giroux, Catal. Rev. 2007, 49, 141-196).